Printers typically have a printhead mounted on a carriage that scans back and forth across the width of a sheet of paper, as the paper is fed through the printer. Fluid from a fluid reservoir, either on-board the carriage or external to the carriage, is fed to fluid ejection chambers on the printhead. Each fluid ejection chamber contains a fluid ejection element, such as a heater resistor or a piezoelectric element, which is independently addressable. Energizing a fluid ejection element causes a droplet of fluid to be ejected through a nozzle to create a small dot on the paper. The pattern of dots created forms an image or text.
Hewlett-Packard is developing printheads that are formed using integrated circuit techniques. A thin film membrane, composed of various thin film layers, including a resistive layer, is formed on a top surface of a silicon substrate, and an orifice layer is formed on top of the thin film membrane. The various thin film layers of the thin film membrane are etched to provide conductive leads to fluid ejection elements, which may be heater resistor or piezoelectric elements. Fluid feed holes are also formed in the thin film layers. The fluid feed holes control the flow of fluid to the fluid ejection elements. The fluid flows from the fluid reservoir, across a bottom surface of the silicon substrate, into a trench formed in the silicon substrate, through the fluid feed holes, and into fluid ejection chambers where the fluid ejection elements are located.
The trench is etched in the bottom surface of the silicon substrate so that fluid can flow into the trench and into each fluid ejection chamber through the fluid feed holes formed in the thin film membrane. The trench completely etches away portions of the substrate near the fluid feed holes, so that the thin film membrane forms a shelf in the vicinity of the fluid feed holes.
One problem faced during development of these printheads is that the thin film membrane and the orifice layer form a composite, which when subjected to stress can crack. When the composite is placed under stress, the thin film membrane, which is the stiffer of the two components, bears the majority of the stress. Thus, when the printhead is flexed or otherwise stressed, either during assembly or operation, the thin film membrane, particularly, in the shelf portion which overlies the trench, can crack. Cracking in the thin film membrane causes reliability problems with these printheads. The problem of flexure and stresses is exacerbated in longer printheads, which typically have larger trenches.